Try shining a torch at an angle close to the screen when the phone is on and check if any images are visible.
They will be very faint if they are there at all so trying this in a darkened room may help.
If you can detect images then there is a backlight problem.
Since you have replaced the screen twice it appears to be on the systemboard.
Most phones have a fuse that feeds the backlight power to the screen.
Unfortunately I don't know where it is located on your phone and I can't find any schematics either.
You could try checking components located near the display flex cable connection on the board. Just a thought.